1. Field of the Invention
This invention relates to a photomask structure, and more particularly to a structure of double-sided photomask.
2. Description of Related Art
A photomask is a key tool in photolithography process; it is used to transfer a desired pattern for fabrication. From this, it can be seen that the photomask plays an important role in the semiconductor fabrications. The photomask is composed of a flat transparent substrate and a light-shielding layer with a pattern, such as a circuit configuration, on the transparent substrate. FIG. 1 is a schematic, cross-sectional view of a conventional photomask. A transparent substrate 100 includes, for example, quartz or glass. A light-shielding layer 102, including, for example, chromium or metal, with a thickness of a few hundred Angstroms is formed over one surface of the transparent substrate 100. An exposing region 170, which is not covered by the light-shielding layer 102, exposes the transparent substrate 100. In order to reduce light reflection during an exposure process, an anti-reflection layer 106 with a thickness of about 200 .ANG. is usually formed over the light-shielding layer 102.
FIG. 2A is a bottom view schematically illustrating a conventional line photomask. In FIG. 1 and FIG. 2A, the FIG. 1 is the cross-sectional view taken along the line I--I in FIG. 2A. A metal line 102a and an exposing region 170a correspond to the light-shielding layer 102 and the exposing region 170, respectively. The photomask shown in FIG. 2A is used for patterning a metal layer on a semiconductor substrate (not shown). FIG. 2B is a bottom view schematically illustrating a conventional plug photomask. In FIG. 1 and FIG. 2B, a cross-sectional view taken along the line II--II in FIG. 2B is shown in FIG. 1. A light-shielding layer 102b and a plug opening 170b correspond to the light-shielding layer 102 and the exposing region 170, respectively.
As the integration of an integrated circuit (IC) device is increased, a photolithography technology with high light resolution is required to achieve precise fabrication of the IC device. One solution proposes using a light source with shorter wavelength to meet this high light resolution requirement. A krypton fluoride laser is an example of an ultraviolet source with a wavelength of 2480 .ANG. for exposure uses. However, a light source with a shorter wavelength can increase the light resolution but cause depth of focus (DOF) to be insufficient. Another solution to the need for high light resolution is to use a PSM in the photolithography process. The use of PSMs has become a trend and so manufacturers endeavor to devote a great deal of resources to the research and design PSMs.
A PSM uses a shifter layer formed over a typical photomask, in which the shifter layer can invert the wave phase of a light ray. When the PSM is exposed, the light rays that pass through the shifter layer have an inverted wave phase, which enables them to interfere with the other light rays. This results in a better pattern resolution in the patterns exposed on a semiconductor wafer. Even though fabrication of a PSM is complicated, the PSM has an advantage in that there is no need of a new light source to increase the pattern resolution due to a modification on a typical photomask.
FIG. 3, FIG. 4A, FIG. 5, and FIG. 6 are the cross-sectional views schematically illustrating four conventional phase shifting masks.
An alternating PSM shown in FIG. 3 is used in a photolithography process. A transparent substrate 300 includes, for example, quartz. A light-shielding layer 302 including, for example, chromium metal is formed over one surface of the transparent substrate 300. Several exposing regions 308, which are not covered by the light-shielding layer 302, are formed sequentially in the cross-sectional view. An anti-reflection layer 306 including, for example, CrO.sub.2 is formed over the light-shielding layer 302. A shifter layer 304 including, for example, MoSi.sub.Z O.sub.X N.sub.Y is formed over the transparent substrate 300 by filling alternating exposing regions 308. The shifter layer 304 can shift a light wave phase by a shift angle of 180.degree.. The thickness of any shifter layer is typically set to have a shift angle of 180.degree. to the exposing light source. In this arrangement of the shifter layer 304, the light rays passing through the exposing region 308 not filled with the shifter layer 304 interfere with the light rays passing through the shifter layer 304 at a critical region between the 0.degree. phase light and the 180.degree. phase light. Thus, a subtraction of the light wave amplitude occurs at the critical region, which is also called a zero point because the light intensity there is zero after amplitude subtraction. Resulting light intensity, obtained by taking the square of the light wave amplitude, has better light contrast so that pattern resolution is increased.
FIG. 4A is a cross-sectional view schematically illustrating a conventional half-tone phase shifting mask. FIG. 4B is a schematic bottom view of a conventional half-tone PSM (HTPSM). A cross-sectional view taken along the line of III--III is shown in FIG. 4A. In FIG. 4A and FIG. 4B, a shifter layer 404 and a number of openings 470 are formed on one surface of a transparent substrate 400. The openings 470 are called a hole pattern, which is commonly used in the photolithography process to pattern a contact opening (not shown) on a semiconductor substrate such that the patterning has a better depth of focus (DOF). The shifter layer 404 has a transmission coefficient of about 3-10% and can invert the light wave phase. When light is incident on the HTPSM, the light passing through the shifter layer 404 has a negative light wave amplitude due to inversion of the light wave phase. As described in FIG. 3, light wave amplitude compensation occurs at the critical region between the 0.degree. phase light and the 180.degree. phase light. Thus, the light intensity contrast is increased so that the pattern resolution is accordingly increased.
A rim PSM is shown in FIG. 5 and FIG. 6. The rim PSM also uses a shifter layer to produce zero points in the pattern so that the pattern resolution is increased. In FIG. 5, the rim PSM is based on a transparent substrate 500. A shifter layer 504 is formed on one surface of the transparent substrate 500 with an exposing region 508, which exposes the transparent substrate 500. A light-shielding layer 502 is formed over the shifter layer 504 but exposes the rim of the shifter layer 504. Then an anti-reflection layer 506 is formed over the light-shielding layer 502. The properties of the shifter layer 504 and the anti-reflection layer 506 are the same as those of the previous descriptions. FIG. 6 shows another rim PSM, which is similar to the one shown in FIG. 5 except that the order of the shifter layer 604 is reversed. The shifter layers 504 and 604 produce zero points to increase the light intensity contrast.
However, as the integration of an integrated circuit (IC) device is increased, it is necessary to reduce device dimension and interconnect line width. In order to achieve high integration, a precise control on a critical dimension of IC devices is required to maintain a sufficient yield rate. The conventional photomasks are more and more incapable of obtaining precise control of the critical dimension. IC fabrication is therefore more difficult and is limited by the photomasks.